If the number of derivatives in the polynomial model of six or
greater, the structure of the propagator is more complicated than
in the fourth-order theory. First of all, the massive poles can be
either real or complex. In the last case, they emerge in the complex
conjugate pairs. There is an important theorem about the structure
of the propagator in the case of real poles [1]}. In this
case, instead of (REF ), we meet
\(G_2 (k)\,=\,\frac{A_{0}}{k^2} + \frac{A_{1}}{k^2 + m_1^2}+ \frac{A_{2}}{k^2 + m_2^2}+ \,.\,.\,.\, + \frac{A_{N+1}}{k^2 + m_{N+1}^2},\) 
